On Thu, May 18, 2017 at 11:41 AM, Richard Sandiford
<richard.sandif...@linaro.org> wrote:
> "Bin.Cheng" <amker.ch...@gmail.com> writes:
>> On Wed, May 17, 2017 at 1:37 PM, Richard Sandiford
>> <richard.sandif...@linaro.org> wrote:
>>> "Bin.Cheng" <amker.ch...@gmail.com> writes:
>>>> -/* Calculates cost for having N_REGS registers. This number includes
>>>> - induction variables, invariant variables and invariant expressions. */
>>>> +/* Estimate register pressure for loop having N_INVS invariants and
>>>> N_CANDS
>>>> + induction variables. Note N_INVS includes both invariant variables and
>>>> + invariant expressions. */
>>>>
>>>> static unsigned
>>>> -ivopts_global_cost_for_size (struct ivopts_data *data, unsigned n_regs)
>>>> +ivopts_estimate_reg_pressure (struct ivopts_data *data, unsigned n_invs,
>>>> + unsigned n_cands)
>>>> {
>>>> - unsigned cost = estimate_reg_pressure_cost (n_regs,
>>>> - data->regs_used, data->speed,
>>>> - data->body_includes_call);
>>>> - /* Add n_regs to the cost, so that we prefer eliminating ivs if
>>>> possible. */
>>>> - return n_regs + cost;
>>>> + unsigned cost;
>>>> + unsigned n_old = data->regs_used, n_new = n_invs + n_cands;
>>>> + unsigned regs_needed = n_new + n_old, available_regs =
>>>> target_avail_regs;
>>>> + bool speed = data->speed;
>>>> +
>>>> + /* If there is a call in the loop body, the call-clobbered registers
>>>> + are not available for loop invariants. */
>>>> + if (data->body_includes_call)
>>>> + available_regs = available_regs - target_clobbered_regs;
>>>> +
>>>> + /* If we have enough registers. */
>>>> + if (regs_needed + target_res_regs < available_regs)
>>>> + cost = n_new;
>>>> + /* If close to running out of registers, try to preserve them. */
>>>> + else if (regs_needed <= available_regs)
>>>> + cost = target_reg_cost [speed] * regs_needed;
>>>> + /* If we run out of available registers but the number of candidates
>>>> + does not, we penalize extra registers using target_spill_cost. */
>>>> + else if (n_cands <= available_regs)
>>>> + cost = target_reg_cost [speed] * available_regs
>>>> + + target_spill_cost [speed] * (regs_needed - available_regs);
>>>> + /* If the number of candidates runs out available registers, we penalize
>>>> + extra candidate registers using target_spill_cost * 2. Because it is
>>>> + more expensive to spill induction variable than invariant. */
>>>> + else
>>>> + cost = target_reg_cost [speed] * available_regs
>>>> + + target_spill_cost [speed] * (n_cands - available_regs) * 2
>>>> + + target_spill_cost [speed] * (regs_needed - n_cands);
>>>> +
>>>> + /* Finally, add the number of candidates, so that we prefer eliminating
>>>> + induction variables if possible. */
>>>> + return cost + n_cands;
>>>
>>> It looks like the return is mixing units. Would it work to return
>>> a <cost, n_cands> pair instead, and use lexicographical ordering?
>> Hi Richard,
>> It just penalizes the cost by the number of candidates, rather than
>> returns n_cands to caller. Actually that information is available all
>> the time in ivopts_data structure.
>
> Yeah, but what I meant was: "cost" seems to be measured in abstract
> instruction units, while "n_cands" counts a number of variables.
> It doesn't seem to make sense to add them together.
Yes, unfortunately GCC adds up different costs and compares it together.
>
> If the idea is to use n_cands as a tie-breaker when the instruction
We don't have such cost model which different costs could be
applied/compared separately, also we don't have obvious break
conditions. Sometimes the "instruction cost" is lower but results in
worse code because register cost is higher; but sometimes the opposite
holds. LLVM has such separated cost models I guess?
Thanks,
bin
> costs are the same, then lexicographical ordering of pairs would give
> that without mixing the units.
>
> Thanks,
> Richard
